Technologies using an unmanned aerial vehicle (UAV) in surveying are publicly known. The UAV used in these technologies may be mounted with a location measuring device using a global navigation satellite system (GNSS) (called a “GPS receiver”), an inertial measurement unit (IMU), an altimeter, and a camera. Such a UAV may be made to photograph the ground while flying along a predetermined path in aerial photogrammetry or in other surveying.
The UAV can locate its own location using the GNSS, but this locating is independent positioning having a positioning accuracy of approximately 1 meter in a horizontal direction and approximately 3 meters in a vertical direction, which does not achieve the accuracy required in photogrammetry. A UAV may be mounted with a more highly accurate location measuring device that can perform relative positioning using a GNSS. However, this idea is difficult to apply to a general purpose UAV in consideration of weight and electric power consumption of the device. To solve these problems, a total station (TS) may be used to track a UAV in flight and locate the UAV by using its laser distance measuring function (for example, refer to US2014/0210663).
This method of tracking the UAV by the TS uses an automatic target-tracking function of the TS. In this technique, searching laser light is used to capture and track the UAV. The UAV has a reflective prism that reflects the searching laser light back in the incident direction, and the TS detects light reflected from the reflective prism to track the UAV.
The tracking of the UAV by the TS is normally performed as follows. First, a UAV placed on the ground before its flight is captured and is locked on by a TS. Then, the flight of the UAV is started, and the flying UAV is tracked by the TS. In this method, an obstacle between the UAV placed on the ground and the TS may prevent the initial lock-on in the initial stage. Moreover, reflected light may not be sufficiently obtained from a prism of the UAV on the ground because the prism serving as a target is normally attached to the bottom of the UAV.
To solve these problems, the UAV may be made to hover immediately after starting its flight to help the TS easily capture the UAV. This method requires some extra time for hovering to allow the TS to reliably capture the UAV. However, the hovering during the set extra time consumes battery power of the UAV.
Moreover, this method may fail to allow the TS to lock on the UAV when the UAV is displaced from a specified hovering position because of being displaced by wind or because of other reasons.